A number of regulatory reactions such as chromatolysis after axon injury, retrograde transneuronal changes, acute glial reactions and growth regulation according to the size of the peripheral field imply the existence of intracellular communication from the axons and/or terminals to the parent cell nucleus. Retrograde intraaxonal transport may provide the means for this communication. We propose to study the retrograde intraaxonal transport of exogenous proteins such as horseradish peroxidase from the region of the axon terminal to the neuronal cell body in the chick and newt visual systems . We shall analyze by light and electron microscopy and biochemical techniques 1) the cytological organelles involved in the transport, 2) the development of the transport phenomenon in embryonic stages of the chick, 3) the effects of injury or of alteration in the physiological state of the neuron on the retrograde transport of proteins, and 4) the possibility of applying additional markers to the further refinement of the retrograde transport technique as a tool for studying neuronal connectivity in the nervous system.